The present invention relates to an operating method for what is termed a silent discharge lamp. This is understood as a type of discharge lamp in which what are termed dielectrically impeded discharges are employed to generate light. The discharge is dielectrically impeded owing to a dielectric layer between the discharge medium of the discharge lamp and at least one of the electrodes. Silent discharge lamps per se are prior art and will not be explained here in detail.
The present invention is based on an operating method, developed by the same inventors, for pulsed coupling of active power into a quiet discharge lamp. Reference is made in this regard to WO94/23442, whose disclosure content is hereby incorporated by reference. The operating method described there forms the basis of the invention described below. It is of paramount importance that what are termed dead times without substantially coupling active power are inserted between individual pulses if active power is coupled into the discharge lamp, and the length of these dead times is dimensioned up to a new pulse which couples active power such that a specific type of discharge described in the cited application and having a particularly high discharge efficiency is formed. The dead times may not be too long for this purpose, because each active-power pulse is then to be evaluated as a new ignition, as it were, and the absence of a connection between the individual active-power pulses renders it impossible to achieve good efficiency, a sufficient lamp power or else good temporal and spatial stability. If the dimensions of the dead times between the active-power pulses are, on the other hand, too short, filamentous discharges form which exhibit poor efficiency and, moreover, a poor temporal and spatial stability.
An invention of the same inventors already filed as an application proposed an operating method and a ballast for a silent discharge lamp with the aid of which the outlined pulsed operating method of WO94/23442 can be implemented particularly effectively. The associated patent application with the file reference 198 39 329.6 has not yet been published at the application date of the present invention, but forms a technical basis for the invention explained below. The disclosure content of this second prior application is therefore also referred to completely.
In particular, it was proposed in this prior application to use a ballast in accordance with the forward converter principle, in which a voltage pulse is impressed from a primary circuit via a transformer into a secondary circuit containing the discharge lamp, and leads to an ignition (termed forward ignition below) in the discharge lamp. The operating method is designed in this case such that after the forward ignition in the discharge lamp an oscillation is set up in the secondary circuit by means of which the charge effecting the external voltage across the discharge lamp, which has previously effected the forward ignition, is removed from the discharge lamp. Thereupon, the remaining internal counterpolarization can lead to a back ignition in the discharge lamp. Reference may be made to the cited application for the details of this basic principle.
In particular, it has already been described in the cited application as a preferred case that the temporal spacing between forward ignition and back ignition is so short that it is not to be regarded as dead time in the meaning of the pulsed operating method. Thus, the abovementioned dead times occur between in each case a back ignition and the forward ignition following thereupon, but not between this forward ignition and the back ignition following thereupon. The following also proceeds on this basis. The operating method described in the second cited application had been developed with the aim of achieving a favorable overall compromise with regard to the power efficiency, the overall volume and overall weight of the associated ballast, and the production costs, service life and failure frequency.
The present invention is based overall on the technical problem of further improving the described operating method according to the forward converter principle. In particular, it is to be possible to operate with the highest possible lamp powers in conjunction with small overall volume and overall weight and good efficiency.
According to the invention, it is provided for this purpose in accordance with claim 1 that in the described operating method an inductance governing the temporal variation in a current through the transformer is varied temporally within a period including a forward ignition and a back ignition such that the altered inductance is substantially larger in an initial phase of the impression of the voltage pulse which leads to the forward ignition than in at least a portion of the back ignition phase, in which the charge is removed from the discharge lamp after the forward ignition and the back ignition is performed.
The invention likewise aims at a ballast designed for this operating method, and at an illuminating system comprising such a ballast and a silent discharge lamp.
The following findings are fundamental in this case: the temporal behavior of the change in the external voltage across the discharge lamp is important for the physical nature, and thus also the efficiency of the silent discharge in the discharge lamp. In particular, it has emerged in this case that excessively large pulse widths should not be selected for ignition in the pulsed operating method. The special efficiency of the pulsed operating method is based, rather, on the fact that a dead time starts again after a relatively short pulse in the coupling of active power.
Consequently, the voltage pulse across the lamp, and therefore also the associated primary current pulse in the transformer must be relatively short.
In particular, the back ignition leads to a more efficient and more complete conversion of the energy stored in the secondary circuit the faster the secondary circuit swings back in the case of the half wave leading to the back ignition and during the back ignition, that is to say after the reignition as a consequence of the internal counter polarization. The aim is therefore to select the natural frequency or speed of the secondary circuit to be as high as possible. The inductance given in the secondary circuit by the transformer plays a substantial role in this speed.
On the other hand, however, it has also emerged that the discharge physics of the forward ignition can, in turn, be unfavorably influenced by excessively steep rising edges of the voltage at the start of a pulse leading to a forward ignition, and thus also by excessively steep rising edges at the start of the primary current rise. Evidently, the situation is that the occurrence of the discharge right at the beginning of the field build-up should favorably still be allowed sufficient time to prepare an optimum form of the discharge structures rendered possible by the pulsed operating method. An excessively low transformer inductance could give rise here to unfavorably steep rising edges. This, in turn, could worsen the efficiency of the discharge. If it is ensured by a sufficiently large primary circuit inductance that the forward ignition has a basic physical form suitable for very high efficiency, there will be no further fundamental subsequent change in this basic fact owing to the speed of the voltage rise in the primary and/or secondary circuit. Specifically, the still remanent residual ionization of the last back ignition is then preimpressed suitably for the new ignition by the electric fields building up.
However, the inductances caused by the transformer in the primary circuit and in the secondary circuit cannot in principle be selected entirely independently of one another. Consequently, the invention provides a temporal variation in at least one of the inductances governing the currents through the transformer.
In particular, the aim in this case is that no excessively low inductance be present in the primary circuit during the preparation of the forward ignition, that is to say in the initial phase of the primary current rise. On the other hand, the secondary circuit inductance is to be relatively low, at least in relation to a portion of the back ignition phase comprising the preparation of the back ignition and the back ignition itself. It is not stipulated with complete accuracy in the basic definition of the invention when precisely the temporal variations in inductance take place. A full freedom of choice in this regard does not exist for all embodiments of the invention.
On the one hand, such temporal variations in inductance can result from switching in a further inductor in a temporally variable fashion. A prescribed transformer inductance is reduced in this case by connecting a further inductor in parallel, and is increased by a connection in series. This can happen in principle in the primary circuit and/or in the secondary circuit. An appropriate connection in the primary circuit is technically simpler in this case. It is possible in the same way to use transistor switches as switching elements, as is provided for the clocking in the primary circuit in accordance with the explanations in the cited second prior application. Such transistor switches can be controlled in this case synchronously with the, and also by the same control device as for the clocking of the primary circuit current, it being possible for the switching instants to be selected freely in principle.
On the other hand, however, a particularly preferred aspect of the invention relates to an, as it were, automatically temporally variable design of the inductance by using the transformer in a saturation mode which exceeds drive levels generally normal for power transformers. This means that the transformer is preferably designed such that not only does it just go into saturation xe2x80x9cin a tolerated wayxe2x80x9d in specific operating phases at the edge of its modulation, but that a substantial portion of its modulation already lies in the saturation region. There is a substantial reduction in the transformer inductance in the saturation region in this case owing to the sizeable reduction in the relative permeability of the core material occurring during the saturation of the transformer core.
Over and above this, it has emerged from the inventors"" experiments that the transformer losses constitute a substantial problem area in the case of a further power rise in conjunction with a prescribed magnitude of the ballast. The practical effect of this is that, starting from a certain power, the thermal losses in the transformer lead to an intolerable deterioration in efficiency and to thermal instability.
The conventional consequence for the person skilled in the art from the fact that the transformer losses increase with higher drive level would be an enlargement of the transformer, in order in this way to be able to reduce the drive level. Specifically, in the case of power transmission the general rule of thumb of avoiding relatively high drive levels of the core materials above 150 mT applies, in order to keep the losses manageable. The point is that the volume-specific magnetic losses in the core materials increase very strongly with increasing drive level. Moreover, they also have a frequency dependence which is, however, of no further interest here. With the ferrite materials normally used, the saturation region is still remote in the case of 150 mT, as follows from the quantitative considerations illustrated in the exemplary embodiments.
The invention here pursues exactly the opposite avenue, because it has emerged that the transformer losses are certainly manageable in the case of a very intensive degree of saturation of a comparatively small transformer. Finally, the transformer losses occur substantially in the hysteresis region of the transformer core. Starting from a drive level of the transformer which approaches the saturation region, these hysteresis losses then virtually no longer increase. On the other hand, a transformer with a correspondingly low volume can be used due to a very strong degree of saturation of the transformer. Although the transformer losses are thereby high with reference to the core volume, they are not excessive in absolute terms, because of the small core volume. It has emerged overall that, together with the further improvement in the discharging efficiency, it is possible to achieve a rise in efficiency, while it is possible nevertheless in this case for the overall volume and overall weight of the ballast, which is essentially determined by the transformer, to be substantially reduced.
It may be remarked at this point that the discussions in this description and the wording of the claims applies in the same way, of course, to the use of two or more transformers instead of a single one. In technical terms, this constitutes only a subdivision of the transformer, but not a change in principle.
The drive level of the transformer is given in this case by the magnetic field in the transformer core.
Consequently saturation occurs chiefly during the back ignition phase, because here relatively large secondary currents can flow without the accompaniment of correspondingly large primary currents. Given sufficiently large secondary currents during the back ignition, saturation occurs, in particular, as early as at the beginning of the back ignition, so that the secondary circuit can swing back quickly and decisively because of the reduced natural frequency. Relatively large primary currents and secondary currents occur simultaneously during the forward ignition phase. The secondary currents are directed such that they attenuate the positive time derivative, producing them, of the magnetic induction, that is to say they partially compensate the magnetic induction in the case of a positive time derivative by an appropriate counterfield. It is therefore not stipulated within the scope of invention whether saturation of the transformer occurs in the forward ignition phase. This can certainly happen through a correspondingly high-resistance impedance of the load in the secondary circuit and, consequently, a low strength of the secondary currents, that is to say a weak compensation effect. However, it is not necessary for the invention. In any case, the saturation effect will not be present right at the beginning of the forward ignition phase, since even the primary currents per se are too small there. By way of illustration reference may be made to the current profile curves explained in the exemplary embodiments.
A further desired effect can also occur in connection with the already explained reduction in the transformer inductance in the secondary circuit and with the consequently increased speed of the secondary circuit. Not only does the transformer inductance determine the speed of charge-reversal operations in the secondary circuitxe2x80x94together with the secondary circuit capacitance, usually defined substantially by the discharge lamp, and the ohmic resistances in the secondary circuit, it is important for the total impedance of the secondary circuit. In many cases, the transformer inductance is the decisive variable in this case. A reduction in the transformer inductance in the secondary circuit therefore entails a marked reduction in impedance in the secondary circuit, and thus the possibility of relatively large lamp currents during the back ignition.
It remains to remark with reference to the cited prior application that the invention described here also preferably provides reducing a residual magnetization at the transformer with the aid of the back ignition. It was argued there that saturation of the transformer had to be feared without a reduction in this residual magnetization. However, what was meant there was a situation in which the amounts of energy remain permanently in the secondary circuit (specifically in accordance with the residual magnetization) or are displaced to and fro between the primary circuit and secondary circuit, without actually being converted in the discharge lamp. Such amounts of energy certainly occur as outputs in the ballast, and so the latter must be appropriately designed, but they do not increase the power of the lamp. They are therefore to be avoided as far as possible. The saturation of the transformer aimed at within the scope of the invention present here relates, however, to a saturation which is always being built up anew with each operating cycle, that is to say is associated with energies and/or outputs which are transported from the primary circuit into the secondary circuit and, as far as possible, into the discharge lamp. A state of saturation is therefore not disadvantageous per se, as already explained above.
It is also preferred in this invention that the secondary circuit is isolated as a resonant circuit by electrical isolation from the primary circuit after the forward ignition. It is preferred in this case that the primary side of the transformer is opened after the forward ignition, the primary circuit current therefore being completely switched off. Furthermore, irrespective of the resonant circuit properties, it is a preferred aspect of the invention that the primary circuit current is virtually zero during the back ignition.
Reference is made in this regard to the explanations concerning the saturation states of the transformer in the exemplary embodiments.
It is preferable to provide for switching off the primary circuit current a switch, in particular a transistor switch, which opens the primary side after the forward ignition, performed according to the forward transformer principle. Thus, with regard to the ballast and the illuminating system, the invention is distinguished by this switch and the design of its control device, as well as by the design of the transformer or by another device for achieving the temporal variation in the inductances.
According to the invention, the most favorable instants for interrupting the primary circuit current lie in a range in which the primary circuit current would exhibit a minimum if there were no interruption. Specifically, were the primary circuit to remain closed after the forward ignition, the primary circuit current would exhibit an intermediate minimum after the maximum during the forward ignition, the quenching of the forward ignition, the ohmic resistance, rising steeply once again, of the discharge lamp, and the corresponding current decrease. The primary circuit current would then rise with time again in accordance with the properties of the transformer. This minimum is an advantageous switching instant, because owing to the minimum primary circuit current the switching losses in the switching transistor, for example, are also minimal.
A MOSFET with a freewheeling diode is, moreover, particularly suitable as the switch. Thus, even given interruption of the actual primary circuit current, it is advantageously possible for demagnetizing currents (for demagnetizing the transformer) to flow in the primary circuit and, for example, to recharge a storage capacitor of the power supply of the primary circuit. A demagnetizing device would thereby be implemented despite electrical isolation between the primary circuit and secondary circuitxe2x80x94accompanied by the corresponding safety advantages.
Preferred embodiments of the invention relate to preferred quantitative delimitations in connection with the temporal variation in inductance. The first of these variants relates to the transformer inductance in the primary circuit which, preferably at the start of the voltage pulse leading to the forward ignition, that is to say in the presence of very small currents, is at least three times as large, preferably at least five times as large and, with particular preference, at least ten times as large as at least in a portion of the back ignition phase. Consequently, with reference to the instant of the first primary circuit current rise during the forward ignition the saturation effect varies the primary circuit inductance by at least a factor of 3 or 5 or 10. A corresponding statement also holds, of course, for the abovementioned circuitry implementing of the temporal variation in inductance (including without saturation mode).
The second quantitative delimitation variant relates only to the case of saturation and for the purpose of delimitation makes use of the magnetic induction or flux density (B field) in the transformer. The aim in this case is for the magnetic induction to be at least 70% of what is termed the magnetic saturation induction of the transformer as early as in the initial phase of the back ignition, when at an instant the secondary current has reached 20% of its maximum in the back ignition. Values over 80%, even better over 90% and, in the most favorable case, over 95% of the saturation induction are preferred.
The magnetic saturation induction is a technical characteristic of the transformer core, and is, for example, specified by transformer manufacturers. It corresponds to the point of intersection of a tangent at the saturation portion of the magnetization curve, that is to say the graph illustrating the magnetic induction as a function of the magnetic field, with the induction axis, that is to say for a zero field (H=0). Physically, this is therefore the magnetization of the core which can be achieved in the saturation of the core, without the field contribution.
Given core materials which are favorable for the invention, saturation inductions of preferably at least 350 mT result, from which it may be seen that under favorable conditions the transformer is driven far beyond the value of 150 mT mentioned at the beginning.
The speed of the secondary circuit achieved by the measures according to the invention is expressed in a half-value width of the secondary current in the back ignition of preferably less than 800 ns.
The transformer core is preferably closed, and therefore has no air gap (that is to say a vanishingly small air gap in conjunction with multipartite cores or an annular core), and can preferably consist of an MnZn ferrite, the material N87 of the manufacturer EPCOS AG or an equivalent material of another manufacturer being suitable. The saturation induction is approximately 370 to 375 mT in this case.
The operating method according to the invention can be implemented as a push-pull method, the voltage pulses leading to the forward ignition being performed in a bipolar alternating fashion. Thus, respectively rectified voltage pulse impressions for which, of course, the forward ignition and the back ignition in the discharge lamp are nevertheless oppositely directed, are designated as a unipolar method. However, a bipolar push-pull method is advantageous with regard to the alkali ion migration effects (blackening effects) which are unavoidable in principle in discharge lamps. By employing a symmetrically alternating method, these effects are incapable in principle of resulting in damage to the lamp. However, it is to be considered in this case that even the use according to the invention of a back ignition provides a substantial improvement with reference to these problems. However, the forward ignition and back ignition are not necessarily symmetrical, and so residual effects can remain in the unipolar case.
Further preferred electrotechnical details relate firstly to the use of a ceramic multilayer storage capacitor in the power supply of the primary circuit, as has already been set forth in the cited prior application. Secondly, likewise as set forth there, it is preferred to use a center tap of the secondary winding of the transformer as reference potential of the secondary circuit.
As has already been set forth, the invention offers not only an improvement in efficiency but, above all, the possibility of driving relatively large lamp powers with very small and light ballasts. This is of decisive importance for some applications because, specifically, it offers the possibility of installing the ballast at sites at which only limited space is available. For example, a ballast according to the invention could move together with a silent discharge lamp in a photocopier or a scanner in a moving device of the silent discharge lamp, and so relatively long and, in addition, moving lines conducting high voltage can be avoided. Furthermore, there is the possibility of integrating such a ballast in lamp bases such that the discharge lamp can be produced and sold as a unit with an integrated ballast, and can be installed without difficulty by the user, for example, in a monitor. In this regard, the invention provides that the lines between the ballast and discharge lamp have a length of at most 10 cm, even more favorably a value of 5 cm. Furthermore, as mentioned, an integration in the base housing of the discharge lamp is preferably provided. A base housing is understood in general as a housing which is built directly on the discharge lamp and contains the electric connections and, in the case of this invention, also the ballast, in addition